1. Field of the Invention
The present invention relates generally to an apparatus for and method of controlling an air-fuel ratio of an engine according to an output from an exhaust gas sensor.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 1993(H05)-141294 discloses that when supply of a fuel is resumed after a fuel cut-off, alteration of an air-fuel-ratio-feedback-correction coefficient is inhibited for a predetermined period of time after the resumption of the fuel supply.
Inhibition of alteration of the air-fuel-ratio-feedback-correction coefficient as described above will generate a delay in the response of an air-fuel ratio sensor, which in turn causes an excessive increase in the amount of fuel injection, and as a result, it will be possible to prevent the air-fuel ratio from becoming excessively rich.
However, if a transient response of the air-fuel ratio sensor slows down due to deterioration in the performance of the sensor, the alteration of the air-fuel-ratio-feedback-correction coefficient might be started during a delay in detection by the air-fuel ratio sensor. As a result, the fuel injection amount is excessively increased and accordingly, the air-fuel ratio will become excessively rich, leading to degradation in performance of emission control as well as drivability.